1. Field of the Invention
The present invention relates to a spin valve with an improved capping layer structure and, more particularly, to a capping layer structure for a spin valve that improves a magnetoresistive coefficient and thermal stability.
2. Description of Related Art
An exemplary high performance read head employs a spin valve sensor for sensing magnetic fields on a moving magnetic medium, such as a rotating magnetic disk or a linearly moving magnetic tape. The sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90.degree. to an air bearing surface (ABS) which is an exposed surface of the sensor that faces the magnetic medium. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetic moment of the free layer is free to rotate in positive and negative directions from a quiescent or zero bias point position in response to positive and negative magnetic signal fields from a moving magnetic medium. The quiescent position is the position of the magnetic moment of the free layer when the sense current is conducted through the sensor without magnetic field signals from a rotating magnetic disk. The quiescent position of the magnetic moment of the free layer is preferably parallel to the ABS. If the quiescent position of the magnetic moment is not parallel to the ABS the positive and negative responses of the free layer will not be equal which results in read signal asymmetry.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layers are minimized. This thickness is less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces or boundaries of the spacer layer with the pinned and free layers. When the magnetic moments of the pinned and free layers are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. An increase in scattering of conduction electrons increases the resistance of the spin valve sensor and a decrease in scattering of the conduction electrons decreases the resistance of the spin valve sensor. Changes in resistance of the spin valve sensor is a function of cos .theta., where .theta. is the angle between the magnetic moments of the pinned and free layers. This resistance, which changes when there are changes in scattering of conduction electrons, is referred to in the art as magnetoresistance (MR). Magnetoresistive coefficient is dr/R where dr is the change in magnetoresistance of the spin valve sensor from minimum magnetoresistance (magnetic moments of free and pinned layers parallel) and R is the resistance of the spin valve sensor at minimum magnetoresistance. For this reason a spin valve sensor sometimes referred to as a giant magnetoresistive (GMR) sensor. A spin valve sensor has a significantly higher magnetoresistive (MR) coefficient than an anisotropic magnetoresistive (AMR) sensor.
The spin valve sensor is located between first and second nonmagnetic nonconductive first and second read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. The distance between the first and second shield layers is referred to in the art as the read gap. The read gap determines the linear bit density of the read head. When a magnetic disk of a magnetic disk drive rotates adjacent the read sensor, the read sensor detects magnetic field signals from the magnetic disk only within the read gap, namely the distance between the first and second shield layers.
Efforts continue to increase the magnetoresistive coefficient (dr/R) of spin valve read heads. An increase in the magnetoresistive coefficient (dr/R) equates to higher bit density (bits/square inch of the rotating magnetic disk) read by the read head. One way to increase the magnetoresistive coefficient (dr/R) is to improve the performance of the various layers of the spin valve sensor. For instance, it is important that the ferromagnetic layers of the spin valve sensor have a uniform texture so as to promote stability of the magnetization of the layer. Further, the compositions of adjacent layers are important. When a first layer is formed on a second layer the characteristics of the first layer are somewhat dependent upon the characteristics of the second layer. This may be due to the grain structure of the second layer being affected by the grain structure of the first layer or a partial diffusion of the layers at the interface.
It should be understood that the performance of the spin valve sensor can be affected by subsequent processing steps, such as setting the magnetization of the hard bias layers, formation of the second shield layer and hard baking of photoresist layers to form the insulation stack of the write head. A temperature of 230.degree. C.-250.degree. C. for a period of 10 hours is typically employed for hard baking the photoresist layers of the insulation stack. This hard baking is done in the presence of a field which is perpendicular to the ABS so as not to disturb the orientation of the magnetic spins of the antiferromagnetic pinning layer of the spin valve sensor. However, the high temperature annealing typically causes a reduction in the magnetoresistive coefficient (dr/R) of the spin valve sensor due to a change in the texture of the layers or interfacial interaction between adjacent layers.